The present invention relates to a process for isolating noble metals from noble metal-containing materials.
Noble metals are metals which are particularly corrosion-resistant. They include in particular gold and silver, but also the platinum metals. Gold and silver have been used for producing jewelry and coins since antiquity because of their corrosion resistance. The metals platinum, palladium and rhodium, in particular, are frequently present in automobile catalysts. Further fields of use of noble metals are electric and electronic appliances, e.g. circuit boards, chips, anodes and cathodes and also sputtering targets, and also fuel cells.
Noble metals are among the most valuable raw materials, for which reason the recovery of these raw materials from used materials has been carried out for many years. Noble metal recycling itself is a valuable material circuit, but it is difficult to separate the noble metals from the products into which they have been incorporated. There are various types of recycling: for example alloys can be treated as fusible scrap. The elements to be recovered are processed in a melting furnace to give granules or ingots and are subsequently recovered in high purity by silver processes and/or electrolysis or wet-chemical processes. One possible way of recovering noble metal from used materials which cannot be recovered by melting is natural separation. The noble metals are recovered in pure form by means of classical separation such as precipitation, filtering or washing.
In addition, a process in which dross is used as starting material is known for noble metal recovery. This means that the used materials consist mainly of organic materials such as paper or plastic and contain small amounts of noble metals. The materials are burnt in a furnace and the metal-containing ash obtained is upgraded via a melting process or wet-chemical purification.
In automobile catalysts and other exhaust gas purification catalysts, noble metals are likewise used as catalyst metals. The catalysts are usually coated with a noble metal, with the metals platinum, palladium and rhodium being most frequently used because of their catalytic properties. The recovery of the noble metals from the catalyst is generally carried out by firstly separating the catalysts from the housing. The catalyst ceramic is subsequently milled to a powder and the catalyst powder obtained is reprocessed in a manner known per se in a separation facility.
Owing to their economic value, efforts are made to recover noble metals as completely as possible from waste materials, also referred to as used materials or materials of value. The processes known from the prior art are costly and generally comprise a plurality of working steps. In addition, the recovery of noble metals is generally carried out using aqua regia, namely a very aggressive mixture of concentrated hydrochloric acid and concentrated nitric acid which contains nascent chlorine and nitrosyl chloride. This component is both toxic and highly corrosive and promotes fires, so that the use of aqua regia requires strict safety measures.
It was therefore an object of the present invention to provide a process for isolating noble metals, by means of which the noble metals can be obtained in a simple way and in high purity.